Color electrophotographic process

ABSTRACT

A COLOR REPRODUCING PROCESS IS HEREIN DISCLOSED IN WHICH A SERIES OF COLOR TONER IMAGES ARE SEQUENTIALLY DEVELOPED IN SUPERPOSITION UPON THE SURFACE OF A PHOTOCONDUCTIVE PLATE. BETWEEN EACH DEVELPMENT STEP, THE PLATE IS IMAGED BY CHARGING THE PLATE TO A POTENTIAL OF A FIRST POLARITY IN BOTH THE PREVIOUSLY DEVELPED AND NON-DEVELPED REGIONS. THE CHARGE ACCEPTED IN THE PREVIOUSLY DEVELPED REGIONS IS THEN PARTIALLY NEUTRALIZED BY APPLYING THERETO A SECOND CHARGE HAVING A POLARITY OPPOSITE TO THAT OF THE INITIAL CHARGE WHEREBY THE ORIGINAL CHARGE IN THE PREVIOUSLY DEVELOPED REGIONS IS REDUCED TO A LEVEL SUBSTANTIALLY EQUAL TO THE POTENTIAL IN THE NON-DEVELOPED REGION. THE NOW UNIFORMLY CHARGED PLATE IS EXPOSED TO A LIGHT IMAGE CONTAINING ADDITIONAL INPUT SCENE INFORMATION RELATING TO THE NEXT IMAGE TO BE DEVELOPED.

COLOR ELECTROPHOTOGRAPHIC PROCESS Filed Nov 5, 1970 2 Sheets-Sheet 1'SEIJI MATSUMOTO BY I ATTORNEY Aug.29, 1972 MA$AM|H| SATO ETAL I3,687,661

7 COLOR ELECTROPHOTOGRAPHIC PROCESS Filed Nov 5, 1970 2 Sheets-Sheet 2 ll I l I7: I i

V i v5 l I V 13 l I United States Patent 01 fice 3,687,661 Patented Aug.29, 1972 US. Cl. 961.2 7 Claims ABSTRACT OF THE DISCLOSURE A colorreproducing process is herein disclosed in which a series of color tonerimages are sequentially developed in superposition upon the surface of aphotoconductive plate. Between each development step, the plate isimaged by charging the plate to a potential of a first polarity in boththe previously developed and non-developed regions. The charge acceptedin the previously developed regions is then partially neutralized byapplying thereto a second charge having a polarity opposite to that ofthe initial charge whereby the original charge in the previouslydeveloped regions is reduced to a level substantially equal to thepotential in the non-developed region. The now uniformly charged plateis exposed to a light image containing additional input sceneinformation relating to the next image to be developed.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to an electrophotographic process and, in particular, to axerographic color process in which the colorants are placed insuperposition upon a photo conductive plate to produce a color copy of amulti-color original.

Description of the prior art conventionally, in the xerographic art, aphotoconductive plate is first sensitized by applying a uniform chargepotential to the surface thereof. The sensitized plate is then exposedto a light image of an original to be reproduced to selectivelydissipate the charge thereon whereby a latent electrostatic image isrecorded on the plate con taining the original input scene information.The latent image is developed or made visible by applying oppositelycharged electroscopic marking particles, i.e. toner, to the plate wherethe toner particles are attracted into imaged areas in proportion to thedensity of the charge present. That is, images of the original inputscene information that are recorded on the plate as regions ofrelatively high charge density are developed as images of high tonerdensity while those original images that are recorded as regions ofrelatively low charge density are developed as images of comparativelylower toner density.

It has been found that the basic xerographic process, as describedabove, can be adapted to reproduce multicolor originals by theapplication of known subtractive color copying techniques. In one suchcolor process, the original subject matter is initially separated intothree primary color components and each of the color componentsprocessed in sequence upon a photoconductive plate so as to formulatethree complementary toner images thereon. The colorants act alone or inconcert, as when superimposed one upon the other, to recreate theoriginal color input scene information. This superimposed developmenttechnique is advantageous in color copying because it can beconveniently automated and also because it elminates many registrationproblems normally encountered in this type of color copying. However, ithas heretofore been difiicult to accurately record image information onthe regions of the photoconductive plate that have previously beendeveloped. The amount of charge accepted by the plate in the previouslydeveloped regions will generally differ from that accepted in thenondeveloped regions. The electrostatic image recorded during exposurewill therefore contain erroneous input scene information resulting in aconsiderable reduction in the quality of the color copy produced.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to improve color xerography.

A further object of the present invention is to improve superimposeddevelopment in xerography to facilitate the production of high qualitycolor copy.

Another object of this invention is to produce a uniformity of chargeover the entire surface of a previously developed xerographic plate.

These and other objects of the present invention are attained by meansof a xerographic color process wherein a multi-color original isseparated into primary color components and each color componentprocessed in sequence upon the surface of a photoconductive platetoproduce complementary toner images thereon that are superimposed oneupon the other in a manner to faithfully reproduce the original. Betweeneach development step, the plate surface is initially charged to apotential of a first polarity and the initial charge then neutralized inthe previously developed regions to reduce the charge density to a levelsubstantially equal to that of the charge found in the non-developedregions. The now uniformly charged plate is then exposed to a lightimage relating to the next color component to be developed to accuratelyrecord color input scene information thereon.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thepresent invention as Well as other objects and further features thereof,reference is had to the following detailed description of the presentinvention to be read in connection with the accompanying drawingswherein:

FIG. 1 illustrates the condition of two images that have been ideallyformulated one upon the other;

FIG. 2 depicts the condition of a superimposed image as a result of thedifferences in charge acceptance found in a previously developed regionand non-developed region on the plate;

FIG. 3 is a graphic illustration showing the charge acceptance curvesfor previously developed and for nondeveloped regions on the plate;

FIG. 4 also is a graphic illustration showing typical light decaycharacteristic curves for previously developed and non-developed regionson the plate;

FIG. 5 is a graphic illustration showing the principles of the presentinvention for reducing the charge potential in developed regions on theplate surface to a level substantially equal to the potential in thenon-developed regions;

FIG. 6 is a graphic illustration embodying the teachings of the presentinvention in which the charge potential in the previously developedregion is reduced after the sensitized plate has been imaged.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention relatesgenerally to a xerographic color process wherein colorants, e.g.,toners, are combined in a subtractive color system to reproduce the huesfound in the original to be copied. In this particular process, amulti-color original is broken down into the three primary colorcomponents of red, green and blue using conventional filtering or colorseparating techniques and each component is then xerographicallyprocessed to formulate toner images of cyan, magenta and yellow. Inorder to eliminate registration problems and the like, the toner imagesare herein developed in superposition upon a single photoconductiveplate. In the practice of the present invention, a light image of thered color component is initially employed to expose the sensitizedsurface of a photocnductive plate, and the latent electrostatic imagecreated then developed using a cyan toner. The plate is resensitized,exposed to a light image of the green color component and developed witha magenta toner. Finally the steps are repeated for the blue colorcomponent and the plate developed using a yellow toner. Ideally, as inall substractive color systems, each of the colorants laid down actsalone and/or in concert with the other colorants to selectively absorblight energy so that when the composite toner image is viewed underwhite light it will faithfully reflect the original input scene. Itshould be clear, therefore, that it is extremely important to regulatethe amount, or the density, of each colorant applied to the plate duringthe development steps in order to preserve the color integrity of theoriginal.

Initially, the plate is uniformly charged by means of a corona generatorsimilar to that disclosed by Vyverberg in US. Pat. 2,836,725 and thecharged plate then exposed to a red color component in a conventionalmanner. By use of known development techniques, such as electrodedcascade development, magnetic brush development or the like, the cyantoner image is accurately recorded on the plate surface whereby thedensity of the toner images produced substantially corresponds to thedensity of the original input image.

As illustrated in FIG. 1, the first developed cyan image 13 isformulated directly on the surface of a xerographic plate consisting ofa photoconductive coating 11 placed upon a conductive backing 12. Thesecond, or magenta, image is superposed over the first cyan image.Ideally, the magenta image should accurately record the input sceneinformation provided by the second or green color component in both thepreviously developed and nondeveloped region on the plate. For instance,when an image of uniform input density is recorded on the plate thatpartially overlies a previously developed cyan region and partiallyoverlies a non-developed plate region, the resultant developed magentatoner image should take the form of image 14 shown in FIG. 1. Thedensity of that portion 14 of the image overlying the previouslydeveloped cyan region should be substantially equal to the density ofthe portion 14" overlying the photoconductive plate surface 11.

In practice, it has been found that subsequently developed images ofthis nature do not, in fact, attain a uniformity of density approachingthat of the ideal case described above. As illustrated in FIG. 2, thatportion of the image 14 overlying a previously developed toner region onthe plate surface will attract more toner particles than are attractedinto region 14" overlying previously non-developed regions. As can beseen, depositing an inordinate amount of magenta toner over a previouslydeveloped-cyan image would result in erroneous color information beingrecorded on the plate surface. It has been found that eflect of thisphenomena becomes more pronounced as the densities of the imagesinvolved increase.

The reason for this phenomena can be best explained in conjunction withthe charge acceptance characteristic curves shown in FIG. 3. Thesecurves typify the amount of charge accepted in both the developed andnon-developed regions on the photoconductive plate when charged by meansof a corona generator similar to that disclosed in the previouslymentioned Vyverberg patent.

The typical charge acceptance curve for the bare, or nondeveloped regionon the plate is represented by curve 18. It will be noted that thiscurve rises rapidly to a relatively high voltage and then levels ed asthe voltage approaches the maximum acceptance potential for the plate,that is, the potential level at which the plate will accept no furthercharge. Similarly, curve 17 typically illustrating the charge acceptancein the previously developed region on the plate closely approximatesthat of the bare plate curve but will remain at a higher potentialthroughout the entire charging period, t to t The shaded area be tweenthe two curves represents the potential diiference between the chargeaccepted in the previously developed and the non-developed regions onthe plate. This potential diflierence has been found to range anywherefrom a few volts to several hundred volts depending on the systemsparameters and materials involved.

Although the phenomena is not fully understood at this time, it isbelieved that both the photoconductive plate and a toner image supportedthereon are capable of accepting and storing a charge potential. As aconsequence, two distinct charges are thought to exist in closeproximity in and about in the previously developed regions, theresultant charge in and about these developed regions being greater inmagnitude than the charge found in the nonimaged or bare plate regions.Regardless of the exact mechanism involved, if left uncorrected, theexcessive voltage produced in and about the previously developed regionsleads to improper recording and development of the next subsequent imageto be processed.

The steps of the present invention leading to the removal orneutralization of this unwanted residual charge potential in thepreviously developed regions will be explained in reference to FIG. 5.In the instant process, the previously developed plate surface ischarged in a conventional manner as noted above from time t to t Hereagain, the charge on the bare plate surface is represented by curve 18.The plate is initially charged to a point V wherein the bare platepotential approaches the maximum acceptance potential. During thecharging period, the charge deposited in the previously developedregions will typically follow that of the bare plate curve, however thischarge will be at a slightly higher potential V Following the initialcharging step, the plate is again charged only this time to a potentialof an opposite polarity. A corona generator similar to that disclosed inthe previously mentioned Vyverberg patent can be also used for thispurpose. The opposite charge is applied for a period extending from timet, to which is sufficient to remove or neutralize the residual chargefound in the previously developed regions. It will be noted from thecurves shown in FIG. 5 that during this neutralization period a portionof the charge retained on the bare plate surface is also neutralized.However, because of the nature of the mechanism involved, the excessivecharge retained in the previously developed regions is dissipated longbefore the plate potential is reduced to a level below which developmentcan no longer be maintained. The charge potential in both the previouslydeveloped and non-developed regions is thus brought to substantially thesame potential V prior to imaging the plate preparatory to the nextdevelopment step. Care should be taken during this neutralization periodto obtain a substantial uniformity of charge in both the imaged andnon-imaged areas and to avoid producing local variations in the chargepotential which could effect subsequent imaging and developing steps. Tothis end, a corona generator is employed capable of depositing a chargeupon the once developed plate surface at a rate that is substantiallyindependent of the potential found upon the receiving surface.

After residual or excessive charge has been neutralized, the plate isexposed from time t to time (FIG. 5) in a conventional manner to recorda latent electrostatic image of the next color component thereon. Theelectrostatic latent image is then developed in superimposed positionover the prevoiusly developed image or images. In this case, a secondmagenta image is placed over the existing cyan image. The steps asherein described are then repeated for the blue component which isfinally developed over the other two images using a yellow toner so asto formulate a multi-color copy of the original.

Another embodiment of the present invention is graphically illustratedin FIG. 6. Here, the developed plate is charged from time t to time t inthe manner described above. At time 1 however, the plate is exposed to alight image of the next color component to be recorded on the plate.This step is accomplished between times t and t in FIG. 6 and, forexplanatory reasons, illustrates the effects of exposing the plate to anoriginal image containing little or no input information, as for examplebackground information. The bare plate potential, as depicted by curve18, is reduced to approximately zero potential level. However, thepreviously developed regions retain a residual charge thereon afterexposure as illustrated by the curve 17. It should be made clear,however, that if the plate were exposed to other than backgroundinformation, the bare plate potential would remain at a relatively highlevel. Consequently, previously developed regions, retaining anexcessive or residual charge thereon, would be at a correspondinglyhigher potential. After the exposure step is completed, the plate isthen treated with a neutralizing or opposite charge between times 1 andt wherein the potential in the previously developed and non-developedregions are substantially equalized.

While this invention has been described with reference to the structuredisclosed herein, it is not necessarily confined to the details setforth and this application is intended to cover any modifictaions orchanges that may come within the scope of the following claims.

What is claimed is: 1. In an electrophotographic reproducing process ofthe type wherein a series of images are sequentially developed insuperposition upon a photoconductive member, the method of recording anelectrostatic latent image between each development step includinginitially charging the previously developed photoconductive member to apotential of a first polarity,

applying a neutralizing charge to the photoconductive surface toequalize the initial charge retained upon the developed photoconductivemember in both the developed and non-developed regions, said equalizedcharged potential being of a magnitude sufiicient to support furtherdevelopment, and

exposing the uniformly charged member to a light image to record alatent electrostatic image thereon containing input scene informationrelating to new subject matter to be developed.

2. In an electrophotographic color reproducing process of the typewherein a series of color images are sequentially developed insuperposition upon a photoconductive member, a method of recording anelectrostatic image between each image development step includinginitially charging the previously developed photoconductive member to apotential having a first polarity,

applying a charge having a second polarity to the previously developedregions upon the photoconductive member to reduce the charge in saiddeveloped regions to a level substantially equal to the charge in thenon-developed regions, said equalized charged potential being of amagnitude sufficient to support further development, and

exposing the charged member to a light image to record a latentelectrostatic image thereon containing input scene information relatingto the next image to be developed.

3. The method of claim 2 wherein the image bearing photoconductivemember is initially charged by means of a corona generator.

4. The method of claim 3 wherein said initial charge is reduced in thepreviously developed regions by applying corona of a polarity oppositethat of said first polarity to the developed region on said member.

5. In an electrostatic color reproducing process of the type wherein aseries of color images are developed sequentially in superposition upona photoconductive member, the method of recording an electrostaticlatent image between each developing step including charging thepreviously developed photoconductive member to a potential of a firstpolarity,

exposing the charged member to a light image to record a latentelectrostatic image thereon containing input scene information relatingto the next toner image to be developed,

partially neutralizing the charged retained in the previously developedregions upon said photoconductive member to eliminate the excessivecharge retained in said regions after exposure wherein a uniform chargecapable of supporting further development is retained on saidphotoconductive member.

6. The method of claim 5 whereby the previously developedphotoconductive member is initially charged by means of a coronagenerator.

7. The method of claim 6 whereby said initial charge is partiallyneutralized after exposure by applying corona of a polarity oppositethat of the polarity of said initial gharge to the previously developedregions on said mem- References Cited UNITED STATES PATENTS 3,337,3408/1967 Matkan 96-1 R 3,060,020 10/1962 Greig 96-1.2

3,060,019 10/1962 Johnson 96-].2

3,576,624 4/1971 Matkan 96-1 R FOREIGN PATENTS 1,082,912 9/1967 GreatBritain 96-1 CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R.

